From time to time during the commercial production of ethylene homopolymers and copolymers in a fluidized bed reactor, it is necessary to shut down the polymerization reactor for periodic maintainance and cleaning. In order to re-initiate polymerization following such shutdowns when supported chromium oxide is intended to be used as catalyst in the process, it is necessary to introduce a quantity of such catalyst into the reactor that is many times that required to carry on reaction at a normal rate. However, because such catalyst normally exhibits long induction times, polymerization usually cannot be initiated for at least 4 to 12 hours following introduction of the catalyst into the reactor. Further, once polymerization commences, it ordinarily proceeds very rapidly due to the large amount of catalyst present. Indeed, so rapid is the polymerization (and heat generation) that it presents the possibility of a runaway reaction which results in fouling of the reactor due to fusing of the rapidly-forming polymer and/or sheeting of the polymer on the reactor wall where mixing and heat removal are not as effective as in the center of the bed.
One procedure suggested to reduce the time required to initiate polymerization of ethylene with a supported chromium oxide catalyst, and to reduce the possibility of a runaway reaction, is to commence polymerization with a supported silyl chromate catalyst and then switch to the supported chromium oxide catalyst after polymerization has begun. However, this procedure suffers from the disadvantage that the properties of the initial polymerization product differ from the desired polymer properties, and hence a great deal of waste product must be produced before the desired product can be obtained. As a result, polymerization must be continued for a time sufficient to provide at least 4 to 6 bed turnovers before the desired product can be obtained (i.e., at least the equivalent of 4 to 6 beds of polymer must be removed from the reactor following the initiation of polymerization before polymers having the desired properties can be obtained).